Estimating Structural and Biochemical Parameters for Grassland from Spectroradiometer Data by Radiative Transfer Modelling (PROSPECT+SAIL)

Abstract

As permanent grassland is a large-scale land-use type in Central Europe, grassland inventories are relevant for ecological and agrarian issues. The objective of this study was to assess structural and biochemical grassland parameters (LAI, chlorophyll, water and dry matter contents) from field spectroradiometer data (ASD FieldSpec II) by radiative transfer modelling (PROSPECT + SAIL). Constraints were necessary to compensate the ill-posed nature of model inversion for accurate parameter retrieval. In this context, we found the foliage moisture content to play an important role. After coupling the equivalent water thickness and the dry matter content in a ratio of 4:1, the estimation accuracy for the LAI clearly improved. In terms of LAI, the RMSE decreased from 0.86 to 0.74, and the range of LAI values measured in the field (min = 0.10, max = 5.88) was reproduced exactly with estimates ranging from 0.05 to 5.46. The spectra reconstructed by PROSPECT + SAIL using the inverted parameter estimates coincided well with the measured spectra. Nonetheless, obtained canopy chlorophyll contents tended to be too high. When using ground measured chlorophyll data for spectra generation, simulated reflectances were clearly higher in the visible domain than the measured ones. This is partly attributed to the chlorophyll absorption coefficients of PROSPECT that may not be true for the majority of grassland plant species in reality. Neverthelesss, the obtained results prove the potential of PROSPECT + SAIL for retrieving structural and biochemical grassland parameters; results may be appropriate for assimilation in the modelling of plant growth or carbon cycle, for example.